A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
One of the goals in device manufacturing processes using a lithographic apparatus is to faithfully reproduce the desired device pattern on the substrate using the patterning device. In order to do that, illumination should be optimized to produce greater image contrast even at or near the resolution limit. Polarized illumination is a tool to achieve better imaging, especially for a lithographic process having a low k1 value, where k1 is a process dependent adjustment factor related to the, achievable resolution R, as given by the Rayleigh criterion of equation (1):R=k1*(λ/NA)   (1)
where λ is the wavelength of the radiation used, and NA is the numerical aperture of the projection system used.